Laser welding method

Abstract: A YAG laser welding is applied to a panel assembly (1) including an outer panel (1a) and an inner panel (1b) manufactured of, for example, aluminum alloy. The YAG laser welding is conducted by any one of (a) emitting one pulse of a YAG laser onto the panel assembly (1) to laser-weld the panel assembly (1) in the manner of spot welding, (b) providing an outer panel (1a) having a slit (11) between portions of the outer panel (1a) where weld beads (3A) are formed, (c) laser-welding the panel assembly (1) using a first YAG laser having a first laser output and a second YAG laser having a second, lower laser output intermittently along a welding schedule line (L), and (d) laser-welding the panel assembly (1) on a line (S) of the panel assembly increased in bending rigidity due to a hemming curvature.

Description: The present invention relates to a laser welding method which can be used for laser-welding aluminum alloy parts. As one example of manufacturing automobile parts of aluminum alloy, the manufacture of aluminum alloy automobile hoods is taken. In the manufacture, an inner panel having a peripheral portion and an outer panel having a hemmed portion are welded at the hemmed portion. If a conventional pressure-type spot welding is applied, the hemmed portion will be seriously deformed under the pressurizing forces from welding tips. This means that it is not desirable to use the conventional spot welding for welding aluminum alloy parts. In place of conventional spot welding, MIG welding or TIG welding could be used. However, because the thermal conductivity and the thermal expansion rate of the aluminum alloy are much greater than those of steel, relatively large deformations will be generated in the weld deposits and the heat-affected zone, and will need repair after welding. To suppress deformation, the utility of laser welding of aluminum alloy parts is being investigated. For example, Publication, "NIKKEI MECHANICAL" published on May 3, 1993 from NIKKEI BP Company Ltd. discloses the application of laser welding to automobile hoods. In the laser welding, a Yttrium-Aluminium -Garnet (hereinafter, YAG) laser is used and is transmitted from a laser generator to a laser torch via optical fibres. The laser torch is moved along a welding schedule line to make intermittent weld beads each having a length of about 20mm. The reason for using a YAG laser is that a YAG laser has a wave length of 1.06 micron and is almost not absorbed by the optical fibres constructed of SiO 2 . However, with the above-described intermittent weld beads there is a problem that the thermal expansion and shrinkage caused in the weld beads and heat-affected zone thereof are relatively large so that relatively large deformations are still caused in the welding portion of the parts. US Patent No: 4,607,150 describes an industrial robot that carries a portable laser welder on its movable end effector. The welder has a hollow compression tube that is axially movable against an anvil to hold a plurality of work-pieces together during welding. The laser beam is brought through the tube from a remote laser generator by means of a light pipe system. An object of the invention is to provide a laser welding method which can suppress deformation or distortion generated in a panel assembly during laser-welding. The invention provides a laser welding method in accordance with claim 1. Embodiments of the invention may include any one of the following methods: (1) laser-welding a panel assembly so that spot-like weld beads spaced from each other are formed; (2) applying laser-welding onto a portion of a panel assembly increased in bending rigidity due to the hemming curvature of a folded over portion of the panel assembly. The method of (1) will be illustrated as a first embodiment of the invention using FIGS. 1, 2, and 6. The method of (2) will be illustrated as a second embodiment of the invention using FIGS. 3 to 5, and 6. The above and other objects, features, and advantages of the present invention will become more apparent and will be more readily appreciated from the following detailed description of the preferred embodiments of the invention taken in conjunction with the accompanying drawings, in which: FIG. 1 is an oblique view of a panel assembly to which a laser welding method in accordance with a first embodiment of the invention is applied; FIG. 2 is a cross-sectional view of the panel assembly taken along line 2-2 in FIG. 1, and is applicable to any one of other embodiments of the invention; FIG. 3 is an oblique view of a panel assembly to which a laser welding method in accordance with a second embodiment of the invention is applied; FIG. 4 is a side view of the panel assembly of FIG. 3 before the laser welding is applied; FIG. 5 is a cross-sectional view of the panel assembly taken along line 12-12 in FIG. 3; and FIG. 6 is an oblique view of a YAG laser welding apparatus used in conducting a laser welding method in accordance with any embodiment of the present invention. FIG. 6 illustrates a laser welding apparatus 13 which can be used for performing a laser welding in accordance with the present invention, and a panel assembly 1 which is to be laser welded. The panel assembly is also shown in FIG. 2. The panel assembly 1 is, for example, an automobile bonnet panel assembly I constructed of aluminium alloy, though the panel assembly is not limited to a automobile hood. The panel assembly includes an inner panel 1b having a peripheral portion and an outer panel 1a having a folded over hemmed portion in which the peripheral portion of the inner panel 1b is held. The hemmed portion of the outer panel 1a and the peripheral portion of the inner panel 1b define a three plate overlapping portion of the panel assembly 1. In the case of an automobile hood, a thickness of the outer panel 1a is about 1mm and a thickness of the inner panel 1b is about 1.4mm. The laser welding is applied to the three plate overlapping portion of the panel assembly 1. A weld bead 3 is formed only between the two plates located on the laser emission side and does not reach the third plate farthest from the laser emission side in order to suppress distortion generated in the hemmed portion of the panel assembly 1 due to expansion and shrinkage of the weld beads and the heat-affected zones. In FIG. 6, the panel assembly 1 is set at a predetermined position on a table 14. A multi-arm robot 15 is disposed in the vicinity of the table 14. A laser torch 16 is coupled to a tip or wrist portion 15a of the robot 15 and is moved by the robot 15 along a welding schedule line L (see, for example, FIG. 1). The laser torch 16 is connected to a pulse YAG laser generator 17 via optical fibres 18 so that a pulse YAG laser is transmitted from the YAG laser generator 17 to the laser torch 16. Operation of the robot 15 is controlled by a robot controller 19 so that the laser torch 16 is relatively moved along the welding schedule line L at a predetermined speed and the laser beam issued from the laser torch 16 is directed toward a portion of the panel assembly 1 to be welded. When a pulse YAG laser is used for the laser welding, it is preferable that the average laser output be set at 500 - 700 watt, that the pulse width is set at 10 - 20 msec, and that a pulse repetition frequency is set at 6 - 10 Hz. Preferably, the laser torch 16 is moved relative to the panel assembly 1 at a speed to 50 - 150 mm/sec. Examples comprising the above ranges and not falling under the scope of claim 1 are not the subject of the invention. More preferably, the laser welding is conducted such that the thickness of the outer panel 1a is about 1mm, that the thickness of the inner panel is about 1.4mm, that the laser output power is set at about 650 watt, that the pulse width is set about 18 msec, that the pulse repetition is about 6 Hz, and that the welding speed is about 90mm/sec. Methods specific to each embodiment of the invention will now be explained. FIGS. 1 and 2 illustrate a panel assembly 1 which has been laser-welded using a laser welding method in accordance with a first embodiment of the invention. The laser welding method in accordance with the first embodiment of the invention includes repetition of the steps of (a) emitting one pulse of a YAG laser from the pulse YAG laser torch 16 onto the panel assembly 1 to laser-weld the panel assembly 1 in a spot-welding manner, and (b) moving one of the laser torch 16 and the panel assembly 1 relative to the other by a predetermined distance while stopping emission of a laser beam from the laser torch 16. The distance is predetermined so that spot weld beads 3 are spaced from each other at a pitch of about 15mm or greater. The reason for the pitch of 15mm or greater is to prevent the spot weld beads 3 from affecting each other in thermal distortion and significantly deforming the panel assembly 1. If the pitch is nearly equal to 15mm, the welding strength between the outer panel 1a and the inner panel 1b is desirably great so that strength and rigidity of the bonnet are assured. Preferably, the size of each spot weld bead 3 is equal to or less than 6mm in a direction along the welding schedule line. The size of each spot weld bead 3 changes according to the laser output and the speed of the laser torch 16. During the step of emitting one pulse of laser, one of the laser torch 16 and the panel assembly 1 is moved relative to the other. When the YAG laser output is about 650 watt, the pulse width is about 18msec, the pulse repetition frequency is about 6 Hz, and the speed of the laser torch is about 90mm/sec, the pitch of about 15mm is thereby obtained. FIGS. 3 to 5 illustrate the panel assembly 1 laser-welded using a laser welding method in accordance with a second embodiment of the present invention. The panel assembly 1 includes the inner panel 1b having a peripheral portion and the outer panel 1a having a hemmed portion 2 in which the peripheral portion of the inner panel 1b is held. The inner panel 1b and the hemmed portion 2 of the outer panel 1a define a three plate overlapping portion of the panel assembly 1 to which YAG laser welding is applied. A portion 21 where the outer panel 1a is bent during hemming is pre-formed to the shape of an arc in cross section before hemming as shown in FIG. 4 so that the outer panel 1a is smoothly bent during hemming. The arc cross-sectional portion 21 and a flat portion 22 of the outer panel 1a defines an intersection S between them. The laser welding method in accordance with the second embodiment of the invention is applied to the intersection S or a portion of the panel assembly 1 located in the vicinity of the intersection S so that an intermittent weld bead 3C is formed on the intersection S or in the vicinity of and along the intersection S. When the weld bead 3C is formed at a position close to but spaced from the intersection S, a distance (1) between a centre line of the weld bead 3C and the intersection S is preferably no greater than about 3mm. Since the intersection S or the portion of the panel assembly located adjacent to the intersection S has comparatively large bending rigidity due to the curvature of the arc portion 21, deformation or distortion caused due to the laser welding is suppressed by the large bending rigidity of the intersection S. Any embodiment of the present invention can be applied to a panel assembly manufactured of aluminium alloy. An apparatus to which the laser torch 13 is coupled is not limited to a multi-arm robot 12 and may be a head moved in a single direction or in X-Y directions. In accordance with the present inventions, an advantage that deformation or distortion generated in a panel assembly due to laser-welding can be suppressed is realised.

Claims: A laser welding method conducted using a YAG laser torch (16) supported by a robot (15), comprising the steps of: (a) providing a panel assembly (1) which is made from aluminium alloy and is free of thermal deformation; and (b) repeating the following steps of (b-1) directing a YAG laser beam from the YAG laser torch (16) onto the panel assembly (1) to laser-weld the panel assembly (1) by forming a spot-like weld; and (b-2) moving the laser torch (16) and the panel assembly (1) relative to one another while the laser torch emission is being directed onto said panel assembly (1) and while the laser torch emission is not being directed onto said panel assembly (1), so that a resultant plurality of spot-like weld beads (3) are formed; characterised in that during said directing a YAG laser beam, one pulse of YAG laser beam is directed from the YAG laser torch (16) onto the panel assembly (1) thereby forming said spot-like weld (3), and said spot-like weld beads (3) are spaced from each other at a pitch of about 15mm or greater, and so that each of said spot-like weld beads (3) is no greater than about 6mm when measured in a direction in which the laser torch (16) and the panel assembly (1) are moved relative to one another. A laser welding method according to claim 1, wherein said panel assembly (1) includes an inner panel (1b) having a peripheral portion and an outer panel (1a) having a folded-over hemmed portion (2) in which the peripheral portion of the inner panel (1b) is disposed, wherein said spot-like weld beads (3) are formed in said folded-over hemmed portion (2), joining said inner panel (1b) to said outer panel (1a). A laser welding method according to claim 2, wherein a portion (21) of the outer panel at which the outer panel (1a) is folded over during hemming being pre-formed in the form of an arc in transverse cross section so that a line (S) is formed at an intersection of the arc portion (21) and remaining flat portion (22) of the outer panel (1a), and wherein said panel assembly (1) is laser-welded intermittently on said line (S), or along but close to said line (S). A laser welding method according to claim 1 or claims 2 or 3, wherein the panel assembly (1) includes a three plate overlapping portion, and a laser output level is selected such that said spot-like weld bead is formed only in the upper two plates of the three plate overlapping portion nearest to a laser emission side with respect to a thickness direction of the panel assembly (1). A laser welding method according to claim 1 or claim 2, wherein the panel assembly (1) is an automobile bonnet constructed of aluminium alloy. A laser welding method according to claim 1 or claim 2, wherein a pulse laser welding is conducted keeping an average laser output level of the pulse laser welding at 500 - 700 watt, a pulse width of the pulse laser welding at 10 - 20 msec, and a pulse repetition frequency of the pulse laser welding at 6 - 10 Hz. A laser welding method according to claim 1 or claim 2, wherein the laser torch (16) is coupled to a wrist element (15a) of the welding robot (15) which is controlled by a robot controller (19) so that the laser torch (16) is moved relative to the panel assembly (1) at a speed of 50 - 150 mm/sec. A laser welding method according to claim 6, wherein said laser welding is conducted under conditions that a thickness of the outer panel (1a) is about 1mm, that a thickness of the inner panel (1b) is about 1.4mm, that a laser output is about 650 watt, that a pulse width is about 18 msec, that a pulse repetition frequency is about 6 Hz, and that a welding speed is about 90 mm/sec.

Inventors: HISANO HIROHIKO, KOBAYASHI HIROAKI, NAKAMURA HISANORI, NISHIO MASAHIRO, YASUMATSU SATOSHI, HISANO, HIROHIKO, KOBAYASHI, HIROAKI, NAKAMURA, HISANORI, NISHIO, MASAHIRO, YASUMATSU, SATOSHI, HISANO, HIROHIKO, C/O TOYOTA JIDOSHA K.K., KOBAYASHI, HIROAKI, C/O TOYOTA JIDOSHA K.K., NAKAMURA, HISANORI, C/O TOYOTA JIDOSHA K.K., NISHIO, MASAHIRO, C/O TOYOTA JIDOSHA K.K., YASUMATSU, SATOSHI, C/O TOYOTA JIDOSHA K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1994
Main classification: B23K26
IPC codes: B23K33, B23K26